Salam.  It really depends on where you want to study.  Most universities accept both IELTS and TOEFL but in my experience some US universities only accept TOEFL.  

some prefer toefl because you will not be required to speak with someone in person and you only record yourself in microphone. some other prefer ielts because you have the questions in front of yourself

i myself prefer Toefl. try one of the toefl simulation softwares and try one ielts exam to find out which one is more suitable for yourself

Hello Christly, Ali said and I quote: "Suppose a college accepts both TOEFL and ITLTS score". That's why I said they are interchangeable. You are right when you point out that some universities only accept TOEFL. The same thing with GRE, you are right, it is required only in the U.S.

Elia, when I applied to some universities in the US, some of them only accepted TOEFL.  Thankfully, these universities do accept certificate of English as a medium of instruction.  

Plus, GRE is only required by US universities.  It's very rare for European unis to require GRE.  

They are interchangeable. What matters for a PHD position is also the G.R.E and you G.P.A, along with publications and good references.